Influence of feedstock powder modification by heat treatments on the properties of APS-sprayed Al2O3-40%TiO2 coatings

Aluminum titanate (Al2TiO5) is a congruently melting compound in the binary Al2O3-TiO2 system, which decomposes below 1200 °C. Its properties (e.g. thermal conductivity, CTE) differ significantly from those of Al2O3and TiO2. Thus it is of special interest to study the stability of Al2TiO5 in the spray process and its influence on the coating properties. A commercial fused and crushed Al2O3-40%TiO2 powder, which was found to be substoichiometric, was selected as the feedstock material for the experimental work, as the composition is close to stoichiometric Al2TiO5. Part of that powder was heat-treated in air at 1150° and 1500°C in order to vary the phase composition, while not influencing the particle size distribution and processability. The powders were analyzed by thermal analysis, XRD and FESEM including metallographically prepared cross sections. A powder having Al2TiO5as the main phase was not possible to be prepared due to inhomogeneous distribution of Al and Ti in the original powder. Plasma spraying was performed with a TriplexPro-210 (Oerlikon Metco) usingAr-H2 and Ar-He plasma gas mixtures with 41 and 48 kW plasma power. Coatings were studied by XRD, SEM of metallographically prepared cross sections, and microhardness HV1. Moreover, the results show a clear influence of the Al2TiO5 content in the feedstock powder on the phase composition of the coatings